Resonance

In this picture, a lego-person swings to and fro, powered only by puffs of air. Can you get the timing just right, and raise the lego person to higher and higher heights?

A swinging pendulum provides one of the best case studies of the periodic motion at the source of all wave motion.

The pendulum has a natural frequency of oscillation; if the puffs of air match that frequency, the lego-person will develop a larger amplitude. If the puffs of air come at the wrong time, they will interfere with the natural motion of the pendulum.

Sound waves do this too. One tuning fork can get another moving, IF their natural frequencies match.

Simulating a Radio Tuner, from Molecular Expressions

Simplified MRI, showing resonance in a simulation from PhET